Multiple‐frequency range imaging using the OSWIN VHF radar: Phase calibration and first results

<jats:p>This paper demonstrates the multiple‐frequency range imaging (RIM) which was implemented recently on the OSWIN VHF atmospheric radar (54.1°N, 11.8°E), Germany. A simple but practical phase calibration method is introduced. We validate the RIM technique and the proposed calibration method successfully by examining various radar experiments with different pulse lengths, mono and coded pulses, evenly and unevenly spaced frequencies, and receiver filter bandwidths. The proposed calibration method not only mitigates the phase imbalance between the echoes received at different transmitting frequencies, but also provides a likely value of standard deviation (<jats:italic>σ</jats:italic><jats:sub>z</jats:sub>) of the Gaussian range‐weighting function for correcting the range‐weighting effect. Moreover, it is found that <jats:italic>σ</jats:italic><jats:sub>z</jats:sub> can be adaptive to signal‐to‐noise ratio when it is employed in practice; this procedure improves the continuity of the imaged powers of RIM around the boundaries of range gates, and an empirical expression has been proposed for this. With the improved power distribution around gate boundaries, we can obtain more available estimates of layer altitudes and exhibit the pass of the layer through gate boundaries clearly. Two observations are shown to demonstrate the maturation of the RIM technique used with the radar: convective cells and double‐layer structures. These atmospheric structures cannot be seen clearly in the original presentation of signal‐to‐noise ratio (or height‐time intensity) of the radar echoes having 150‐m or 300‐m range resolution.</jats:p>